Today's internal combustion engines must meet ever-stricter emissions and efficiency standards demanded by consumers and government regulatory agencies. Accordingly, automotive manufacturers and suppliers expend great effort and capital in researching and developing technology to improve the operation of the internal combustion engine. Turbochargers are one area of engine development that is of particular interest.
A turbocharger uses exhaust gas energy, which would normally be wasted, to drive a turbine. The turbine is mounted to a shaft that in turn drives a compressor. The turbine converts the heat and kinetic energy of the exhaust into rotational power that drives the compressor. The objective of a turbocharger is to improve the engine's volumetric efficiency by increasing the density of the air entering the engine. The compressor draws in ambient air and compresses it into the intake manifold and ultimately the cylinders. Thus, a greater mass of air enters the cylinders on each intake stroke.
With reference to FIG. 1, turbochargers use the exhaust flow from the engine exhaust manifold to drive a turbine wheel 10. Once the exhaust gas has passed through the turbine wheel and the turbine wheel has extracted energy from the exhaust gas, the spent exhaust gas exits a turbine housing (not shown). The energy extracted by the turbine wheel is translated to a rotating motion which then drives a compressor wheel 32. The compressor wheel draws air into the turbocharger, compresses this air and delivers it to the intake side of the engine.
The rotating assembly includes an integral turbine wheel 10 and shaft 11. The compressor wheel 32 is mounted to shaft 11. The shaft 11 rotates on a hydrodynamic bearing system 18 which is fed oil, typically supplied by the engine. The oil is delivered via an oil feed port 21 to feed both journal and thrust bearings. The thrust bearing 59 controls the axial position of the rotating assembly relative to the aerodynamic features in the turbine housing and compressor housing. In a manner somewhat similar to that of the journal bearings, the thrust loads are carried typically by ramped hydrodynamic bearings working in conjunction with complementary axially-facing rotating surfaces of a flinger 40. The turbocharger includes a housing 20 with a cavity 33. The thrust bearing 59 and insert 60 are disposed in the cavity and provide an oil drain cavity 35. Once used, the oil drains to the bearing housing and exits through an oil drain 22 fluidly connected to the engine crankcase.
The traditional approach to mounting a compressor wheel to a turbine shaft is by close fit of concentric cylindrical surfaces (wheel bore to shaft outside diameter). A small clearance minimizes the variation or migration of imbalance during operation. Imbalance can cause destructive failure of bearings due to forces generated and vibratory modes excited. In order to help prevent imbalance migration in traditional designs, the fit between the wheel bore and shaft diameter must be maintained at a very tight tolerance. Accordingly, the tolerances on the wheel bore and shaft diameter must also be very tight. It should be noted that these tight tolerances must be maintained over the entire length of the shaft. Tight tolerances result in higher production costs. Furthermore, the tight fit between the wheel bore and shaft diameter makes assembly of the components more difficult, not to mention disassembly. This approach to mounting a compressor wheel to a turbine shaft does not solve the problem of differential mechanical and thermal growth of the wheel relative to the shaft. For an Aluminum wheel piloted on a steel shaft, differential thermal and mechanical growth may be as much as three times the assembly clearance. Thus, adverse imbalance migration is possible in service.
Another traditional approach to mounting a compressor wheel to a turbine shaft includes creating an interference pilot fit to allow for larger manufacturing tolerances and account for differential thermal growth. With cylindrical pilot lands this approach causes assembly issues. Wheels must be heated or driven onto the shaft by force. The length of the pilot land can make small amounts of runout of the shaft or bore critical. Should the resulting assembly not pass a core balance check, removal of the wheel for re-indexing could result in damage to both the wheel and shaft. For example, turbine wheel materials, such as Titanium, are prone to galling and can seize prior to fully seating. In such cases, scrap costs are very high.
Mounting a compressor wheel to a turbine shaft is further complicated by the need to balance the compressor wheel. Compressor wheel balance correction is traditionally accomplished by metal removal in two planes. The aft plane is corrected by removal of material from the perimeter of the compressor wheel back wall. Scalloping between blades or machining a step pocket in the back wall are two methods used. This material removal is extremely critical to the lifetime of the part as the correction zone can be highly stressed. Thus, removal can have an adverse affect on fatigue life.
The forward correction plane is the nose of the wheel. It is lightly stressed so it can be cut away without significant detriment to function. The essential problem is producing enough back wall correction to minimize scrap without inducing premature failure.
Accordingly, there is a need for structures and methods for accurately piloting a compressor wheel onto a shaft, without the cost of extreme precision machining or the assembly drawbacks of an interference fit. There is a still further need for a design that simplifies balancing a compressor wheel without compromising the fatigue strength of the wheel.